The present invention relates to an adaptive transform encoder for an image signal, and more particularly to an adaptive transform encoder suitable for use with a recording/reproducing apparatus
Data compression methods for a television signal includes transform encoding in which a frame is divided into blocks each comprising a plurality of pixels, orthogonal transform is performed for each block and transform coefficients thereof are encoded. In this transform encoding, in order to improve the data compression efficiency, an adaptive transform encoding method has been proposed in which assignment of a quantization bit for each block is changed in accordance with a frequency component of the image. An example of the adaptive transform encoding method is described in IEEE Trans. Commun. COM 25, 11 (1977), pp. 1285-1292 (Reference 1), in which a two-dimensional discrete cosine transform is used as as two-dimensional orthogonal transform. The two-dimensional discrete cosine transform and inverse transform are shown in formulas (1) and (2) of Papers of The Institute of Electronics and Communication Engineers of Japan, March 1986, Vol. J69-B, No. 3, pp. 228-236 (Reference 2).
The adaptive transform encoding method which uses the two-dimensional discrete cosine transform is briefly explained.
As shown in FIG. 1, the active picture 14 of one field is divided into blocks 15 comprising N (scanning lines).times.N (pixels), and the two-dimensional discrete cosine transform is performed for each block. When an input is given by f(i, j) and a transform coefficient is given by F(u, v), the two-dimensional discrete cosine transform is given by the following expression, where i and j are two-dimensional coordinates of the pixels in the block; ##EQU1##
The inverse transform is given by the following expression; ##EQU2##
As to the transform coefficients F(u, v), F(0, 0) represents a DC component and others represent AC components. An activity index A is defined by a sum of powers of the AC components. Namely, the activity index is given by the following expression, (The activity index A corresponds to the formula (7) on page 1287 of the Reference 2); ##EQU3##
A threshold is set for the activity index A and the blocks are classified by the magnitudes of the activity index and the threshold of each block. Quantizers of different numbers of bits are provided one for each class. Usually, the quantizer is a linear quantizer for the DC component, and a non-linear quantizer for the AC component. A block having a small activity index is quantized with a small number of bits, and a block having a large activity index is quantized with a large number of bits. As a result, an encoding which conforms to a local change of the image can be attained and the compression efficiency is improved.
The activity index varies with the picture pattern of the block. A block which includes many high frequency components has a large activity index. In the prior art mentioned above, the number of bits needed changes with the picture pattern because the assignment of the quantization bits is adaptively changed with the activity index of each block. The block having a fine picture pattern which has a large activity index requires a large number of bits, and the block having a picture pattern which results in a small activity index (for example, background having small two-dimensional change) requires a smaller number of bits.
Digital signals can be recorded by tape recorders and the like. However, the tape running speed or rotation speed of head cylinder can not be easily changed momentarily in the tape recorders and the like. Accordingly, it is very difficult for the digital tape recorder and the like to record signals with a recording rate which always changes.